The present invention concerns the design and method of operation of a device for taking samples from sources of liquid such as, bore holes, sumps, conduits carrying liquid chemicals such as petroleum or alcohols, a stream of water, sewers or effluent channels. It is particularly concerned with the provision of a machine to be used in environments which, by virtue of a susceptibility to flooding or the presence of noxious or flammable gases, are hostile.
Sampling methods known at their simplest include manually taking a sample using a scoop or other such device. Clearly requiring the person sampling either to make repeated returns to the sample point or to remain at the sample point over an extended period of time is both an inefficient use of manpower and, in a hostile environment such as a sewer, potentially dangerous. To overcome this disadvantage a sampling machine capable of operating unattended is desirable. In realising this objective a number of difficulties are to be overcome. When a multiplicity of sites are to be covered but samples are required only infrequently at any given site it is uneconomic to build fixed installations: the sampling machine must therefore be capable of being portable. For the sampling machine to be taken to sites where access is difficult the machine must be compact and rugged.
The major difficulty is the requirement that the machine must be able to operate safely in the presence of liquids and/or potentially explosive gases. Electrically operated systems would have to be comprehensively insulated and isolated from their immediate environment to operate safely. Attaining this degree of insulation and isolation adds significantly to the cost of the device and makes it more difficult to obtain the goals of compactness and portability. In many situations the provision of a supply of electricity is difficult if not impossible. In addition to meeting these requirements the sampling machine must efficiently fulfill its prime purpose of drawing a series of samples over a period of time. To provide valid results it is desirable that each sample is discrete, i.e. that each sample is not contaminated with residues from any preceding sample.
One past attempt at solving the problems addressed by the present invention is disclosed in FR No. 2343239. This machine is an electro-mechanical device having a reversible peristaltic pump driven by a conventional electric motor an, in the principal embodiment, a rotatable delivery tube driven by an electronically controlled stepper motor. In use the liquid drawn by the pump is transferred to the delivery tube and then to one of a number of fixed containers. Since, unlike the system of the present invention, this machine relies upon electricity for motive power and control it is inherently ill-adapted for use in hazardous areas. It is attempted to overcome this problem by sealing the electric motor within a plastic compartment. Since it is necessary for the device's control panel to be accessible this cannot be positioned within the sealed compartment and so each of the switches on the control panel has to be individually sealed. Sealing all the electrical components to the high standards necessary for safe use in the presence of liquids and flammable gases necessarily adds significantly to the cost of manufacture and maintenance of the device and the need to have particular components positioned within sealed compartments contains the layout of the components within the device, limiting the extent to which its overall volume can be reduced.
The sampling machine disclosed in FR No. 2343239 has further major disadvantages in that complete purging of the sampling/delivery system is not possible. Reversing the direction of the peristaltic pump serves to purge the tubes directly connected to the pump but leaves the rotatable delivery tube unpurged. Any residues in the form of liquids or liquid-borne solids within the delivery tube will therefore tend to contaminate subsequent samples.
Another electrically, powered sampling device is disclosed in U.S. Pat. No. 3,795,347. This device uses a two-stage sampling process in which liquid is drawn under vacuum into a closed metering chamber until it rises to a predetermined level within the chamber and then the entire contents of the chamber are expelled under pressure into a storage vessel. Prior to a sample being drawn into the metering chamber pressure is applied to purge residues from the chamber and from a tube linking the source of liquid to the chamber. The device uses an electrically powered pump to pressurise the metering chamber and electrically powered timing and control devices. It therefore suffers all the attendent disadvantages discussed above in relation to FR No. 2343239. Moreover it is impossible for this device to be comprehensively insulated since it relies upon a pair of electrodes being exposed to the sample fluids in the metering chamber. It is therefore not possible to operate this device with complete safety in electrically hazardous environments. The two stage sampling process makes this device operationally less efficient than a device in accordance with the present invention in which the pump acts directly upon the liquids to draw a sample rather than being used indirectly to pressurise or evacuate an intermediate chamber. Since the process of purging can never be totally effective, particularly when liquid-borne solids are present, the use of an intermediate chamber provides an additional source of contamination of samples. Furthermore the tube leading from the metering chamber to the vessel for retaining the sample is not purged by the initial pressurisation of the chamber and so provides a further source of contamination. In all these respects this device contrasts with the present invention which provides a direct and easily purged connection between the source of the liquid and the outputs of the device.